1. Field of the Invention
This invention relates to radio frequency (RF) sputtering apparatus for depositing materials on substrates.
2. Description of the Prior Art
The sputtering of dielectric materials by radio frequency fields has become increasingly important with the development of integrated semiconductor circuit devices. Insulating films of electrically stable, high melting or softening point materials can be applied to suitable semiconductor substrates with better results than are achievable with other processes such as evaporation and chemical vapor deposition. RF sputtering allows glass to be contoured or shaped during deposition by the technique of controlled resputtering. In addition, resputtered glass is not as porous and does not contain as many OH molecules as glasses deposited by other processes.
As is known in the art, the deposition of films by RF sputtering has advantages over DC sputtering; mainly the latter can't be used to sputter insulators without the provision of an electron source to neutralize the target. In addition, RF sputtering apparatus may be used to "planarize" a sputtered film, either during the deposition process or after the non-planar film has been deposited.
The phenomenon of resputtering involves the re-emission of deposited insulative material, such as SiO.sub.2, during the sputter deposition thereof through the effects of attendant ion bombardment of the deposited insulative layer. The application of the principles of RF sputtering to resputtering is disclosed in the article "Re-Emission Coefficients of Silicon and Silicon Dioxide Films Deposited Through RF and DC Sputtering," R. E. Jones et al, Journal of Applied Physics, November 1967, pages 4656. In effect, resputtering is the positive ion bombardment of an insulative film during its deposition.
The prior art has recognized that resputtering improves the quality of sputter deposited film, U.S. Pat. No. 3,661,761 discloses the use of RF sputtering to improve film quality and uniformity. U.S. Pat. No. 3,983,022 discloses that resputtering at a substantially zero deposition rate can be used to planarize films deposited atop uneven surfaces.
While resputtering has been used in the commercial fabrication of integrated circuits for the purpose of improving the quality of sputter deposited film, the extent of use for complete planarization has been limited because of the amount of time necessary to achieve complete planarization of an insulative layer deposited over raised conductive line patterns on semiconductor substrates and other factors.
Sputtering apparatus described by Auyang et al in the IBM Technical Disclosure Bulletin, September 1971, page 1032 has been used for planarization of thin films. However, the control of this apparatus, known as a "Driven" system is complicated by the requirement for adjusting five variable reactances. This requires the services of a skilled technician; as a result driven systems would be difficult to control in large scale manufacturing.
Another well known RF sputter deposition system, now termed a "tuned-anode" system, is described in the article by J. S. Logan entitled "Control of RF Sputtered Film Properties through Substrate Tuning", IBM Journal of Research and Development, Volume 14, pages 172-175, 1970. In this apparatus a tunable L-C network is disposed between the insulated substrate electrode and a reference potential, ordinarily ground. By this means the RF current through the substrate electrode and, therefore, the amount of resputtering from the substrates disposed thereon may be controlled. As noted by Logan, this control of resputtering is required for good edge coverage of the deposited insulator over steps in the substrates. But the amount of resputtering is limited and the tuned anode system has not heretofore been used successfully for planarization.
Usually, before sufficient RF current through the substrate electrode, termed the anode, can be obtained the system becomes unstable. This is evidenced by a sudden increase in RF current from the plasma to the grounded parts of the internal tooling. There is a corresponding decrease in anode current. This effect is illustrated in terms of D.C. bias in FIG. 3 of Logan publication. At the point of instability the plasma density near the chamber wall increases, with a corresponding decrease in plasma-to-chamber wall impedance. This causes a diversion of RF current to the chamber walls thereby reducing the current which is directed to the substrate electrode. One technique for minimizing this problem has been to attempt to reduce the ratio of the area of the grounded wall to the cathode area. This would reduce the amount of current which can be diverted to the wall rather than to the anode. The extent of this solution is necessarily limited due to purely geometrical considerations.